Method of producing powdered steel products



Eugene E. Carlson, Ann Arbor, Mich, and Fritz V. Eenel, Troy, N. Y.,assignors to Federal-Mogul Corporation, Detroit, Mich, a corporation ofMichigan Application June 15, 1954, Serial No. 436,323

3 Claims. (Cl. 148-465) The present invention relates generally to theart of powder metallurgy and more specifically to an improved article ofstainless steel and to an improved method of making same from powderedmetallic ingredients.

The molding of metal powders has been extensively employed in theproduction of complicated shapes of soft metals, particularly iron andlow-carbon steels. The method usually employs a fine iron powder whichis pressed or compacted under high pressure to cold weld the metalparticles and then sintered at a high temperature sufficient to form asolid article. This method is quite satisfactory for soft iron and ironlow in alloying constituents.

If it is desired to increase the density of the final articles, it hasbeen suggested, as shown in Patent No. 2,411,073, to coin theonce-sintered compact and then resinter to relieve the cold-workingstresses.

it has sometimes been the practice to carburize and heat treat the softiron to produce a carbon steel structure. For example, Patent No.2,333,573 discloses a method of making low-carbon steels by powdermolding, the method including the use of carbon-free electrolytic iron,compacting and then sintering in a carburizing atmosphere which would bein equilibrium with a carbon steel of the desired carbon content. In thelatter method,

the sintering is continued until the carbon is homogeneously distributedthrough the compact. While the latter method is quite satisfactory forordinary carbon steels, the presence of chromium, nickel and otheralloytion of the chromium and carbon through the specimen.

Another procedure is disclosed in Patent No. 2,489,839, wherein there isdisclosed the compacting of soft iron at low pressures, sintering,coining the resulting porous soft iron compact at higher pressures,carburizing it in a rich atmosphere above equilibrium to obtain ahighcarbon case on the surface, and finally soaking or homogenizing inan equilibrium carburizing atmosphere (i. e. in equilibrium withthe'final carbon content desired) to distribute the carbon throughoutthe specimen.

When it has been desired to produce articles from high alloy steels,particularly high chromium and high chromiurn-nick'el steels, i. e. thestainless steels, alloy powders having the desired composition have beenemployed. These pre-alloyed powders have been pressed and the resultingcompacts sintered as described above. However, because of the poorcompressibility of these powders, it has not been possible to producecompacts of high density, i. e. above 95% of the theoretical or of aforged article, even if the compacts are coined and resintered. Highdensity inthese alloys is needed for good fatigue resistance. It hasalso not been possible to produce heat-treatable article from thesealloy powders since the usual atmosphere used for sintering the compactsd'ecarburize them effectively. It has not been possible predictably tocarburize stainless steel compacts by the usual cracked ammonia,combustion gas, or hydro gen-hydrocarbon gas mixtures. Cracked ammoniaand combustion gases contain high proportions of nitrogen which stronglyfavors the formation of the austenite phase, the latter then interferingwith carbon difiusion and favoring the production of an extremely hardhigh carbon case on the article. Combustion gases also contain carbonmonoxide, a gas which is easily reduced by chromium. The result'is anarticle of poor properties due to the presence of unreduced chromiumoxides in the piece. A principal disadvantage of the hydrogenhydrocarbonmixtures is that they are diflicult to use and control.

The art of powder molding, therefore, has long sought a process whichwould be capable of producing dense, heat-treatable articles ofstainless steels having the fatigue resistance and other physicalproperties of a wrought article of the same composition.

It has been found, in accordance with this invention, that an article ofstainless steel containing at least 11% and up to 25% chromium and up to2.5% nickel can be formed having a density between 96 and 98% or more oftheoretical and having the tensile strength, hardness, elongation, andfatigue resistance closely approaching that of a forged article of thesame alloy by a process including the steps in the sequence of (1)mixing a pure, deoxidized, low-carbon powdered iron of highcompressibility, powdered chromium, ferrochrome or other powdered formof chromium, powdered nickel or nickel alloy and other powdered alloyingingredients; (2) compacting the mixture by pressing at a pressure of atleast 50 t. s. i. (ton/sq. in.), and preferably at 70 t. s. i. or more;(3) sintering the compact at a temperature between 2,000 and 2,500 F.and preferably between 2,300 and 2,450 F. in a reducing atmospherehaving a dew point below -25F.; (4) coining the sintered compact at apressure of at least 60 t. s. i., and preferably at 70 t. s. i. or more,to obtain a high density, and (5) carburizing the compact at atemperature between 2,000 and 2,300 F., and preferably at temperaturesbetween 2,100 and 2,300 F., in a reducing atmosphere having a low dewpoint and in the presence of solid carbon, the atmosphere employing dryhydrogen to react with the solid carbon and form a carburizing gas.Also, according to this invention, when a stainless steel article havinga relatively thick cross section is desired (i. e. thicker than M3), thecarburized compact is subsequently soaked or homogenized at atemperature between about 2,100 and 2,300 P. in an atmosphere-freemolten bath. Although the carburizing step of this invention does notproduce a hard, high-carbon case, the articles of thicker cross sectiondo not have the uniform carbon distribution so much to be desired in anarticle for rigorous service. The homogenizing step is carried out at ahigh temperature to speed the diffusional process.

The above-outlined process produces a molded stainless steel compacthaving little dimensional change (i. e. can be made to extremely closetolerances), having a density of at least 96 to 98% of theoretical, thestrength, elongationyhardness, and fatigue resistance approaching thoseof a forged article of the same alloy, and with good reproducibility.This process can be employed to mold complicated air foil shapes such asrotor and stator blades for compressors in gas turbines, jet engines,and the like, which articles have heretofore been made only byexpensive, time-consuming, forging, machining, finishing and polishingoperations. The above process is believed to be the first to produce asatisfactory powder molded heat-treatable article of a high temperature,corrosion-resistant stainless steel having good fatigue resistance.

The significance of the above sequence of steps, and the conditionsemployed in'each step, isnot fully understood but is believed to becritical for the production of satisfactory powder-molded articles. of.stainless steel. For example, the use ofan iron powder of highcompressibility rather than an alloy. powder makes possible a, higherdensity. The pro-alloyed powders do not readily weld together on coldpressing and the compacts formed from them have relatively low density.In contrast, the use of a very pure, soft iron produces a compact ofhigherv density in the initial pressing step, and permits a stillfurther reduction in porosity during the coining step; The iron powdermust be low in carbon, i. e. below 0.02% carbon, andlowlin oxygen,i. e.below 0.1% oxygen, in'. order toprevcnt. preferential reaction ofchromium and other alloying metals with oxygen and carbon to form oxidesand carbides which are not readily reduced or dissolved, therebycreating. a carbideor oxide-coated skeleton in the compactwhichresists.compacting during the coining operation.

The atmosphere in the sintering stepis more highly. critical in themethod of this invention due to .the presence of pure chromium or highchrome ferrochrome particles. with the chromium and other metals at suchhigh temperatures and the oxide coatingthus formed would interfere withsintering of the particles and with diffusion of the various alloyingmetals. For this. purpose, the dew point must be below -.2S E. andpreferably is in the range of -35 to -45 F. or lower. Also, the presenceof a reducing atmosphere tends still further to reduce the carboncontent of the compact and facilitates the subsequent repressing orcoining step so asztoobtain a-higher density. The use of" the highersintering temperatures of 2,000 to 2,500.F; is believed necessary toobtain proper diffusion of the high melting chromium, nickel and otheralloying ingredients throughout the iron phase. Also, the hightemperatures reduce the length of the sintering cycle and thus preventtheformation of an excessively large-grained structure.

At any given temperature, there is aminimum sintering period that isrequired for optimum properties.

Any oxygen or moisture would readily react usually is non-hardenable dueto its extremely low carbon content of 0.01 to 0.04%. Suflicient carbonmust, therefore, be introduced to obtain a hardenable composition.Should an atmosphere be employed which has too high a dew point, thecarbide diffusion and structure will be improper and the properties ofthe final compact will be inferior. If the carburizing step is carriedout at too low a temperature, the carbon will not readily diffuse intothe interior of the article, but will form a high car'- bon casecontaining massive chromium carbides on its surface. Such a hard case isunusually stable, since it is very difiicult to decompose the carbideduring any subsequent homogenizing step and. to produce a uniformheat-treatable structure throughout the article.

The carburizing step is carried out using dry hydrogen as a carrier forthe carbon. This is carried out according to the invention by placingthe campact near but out of actual contact with carbon, graphite, andother solid forms of carbon. At the temperatures employed, the hydrogenreacts with thesolid carbon and forms a carburizing gas, the lattertransferring the carbon to the sample, probably in the form of methaneor some cracked, intermediate, reactive product thereof. The resultingatmosphere is not oxidizing in nature, in contrast to anatmosphere whichcontains carbon monoxide. The carburizing effect of this atmosphere isvery uniform and is easily controlled, the carburization penetrating toadvantage resides in extending'the time beyond this period, in. fact,increased grain. growth. obtained. during protracted sintering isgenerally disadvantageous. For

example, when a compact. is. sintered. at. 2,350? F.', its

ultimate tensile strength continues toincrease as the sintering time isincreased up.to aperiodof'two hours and no significant. increase occursbeyond two hours. This would indicate that sintering of. the particlesand diffusion of the ingredients. of the alloy is essentially completein two hours. Of course, with. temperatures less than 2,350 B, longersinteringv times will be required while with higher temperatures,slightly shorter periods will sufice.

The use of a coining stepemploying pressures of 60, 70 and up to 100 t.s. i. or more pressure effects a significant increase in density'inthecompactbut only when the latter has been properly. compacted andsintered. During this coining operation, the carbon content of thecompact must be-very low (below 0.1%)

and preferably below 0.05%, in order to obtain any substantial increasein density. during the coining operation. In the sintering .operationofthis invention, the

already low carbon content is markedly reduced. The sintering andcoining steps canbe repeated'if desired, 'although one such cycleusually produces adensity of 96 to 98% or more of theoretical.

The carburizing step is especiallyimportant. inorder to produce aheat-treatable stainless steel alloy. The product of the compacting,sintering andcoining steps a greater depth without case formation. Theuse of temperatures above 2,000 F. also shortens the carburizing timeand thereby reduces the tendency to form a casehardened skin. As aresult, the carbon so introduced is more uniformly distributedthroughout the specimen and is in a form more easily homogenized.

Whilea total carbon content of 0.08 to 0.15% is generally desirable forstainless steels of about 11-13% chromium, the higher chrome steelsrequire increased amounts to be heat-treatable. At about 25% chromium upto 1% carbon may be required for heat-treatability.

The amount of carbon, per unit of compact surface area, introducedduring the carburization step can be most accurately controlled bycontrolling the time and temperature of carburizing. This is a much morepositive and simpler means of control than in the usual gas carburizingmethod in which the ratio of a carburizi-ng gas such as methane, andcarrier gas such as hydrogen, is largely determinative of the amount ofcarburization. The carbon content introduced in representative times andtemperatures is shown in Figs. 1 and 2 of-the accompanying-drawings.From Fig.- 1, which is a plot of time of carburization, as abscissae,and-lbs. carbon/ sq. in. las ordinates, it can readily be seen that fora 'givendesired carbon-content,- a time of carburization can-bedetermined'by reading horizontally.- The latter figure is useful whereitispossible to operate at any of the three temperatures shown on thegraph. Figure 2, however, is a plot of lbs. (carbon/ sq. in.) x" versustemperature. Using the latter curve, it is possible to determine thetemperature to be used to obtain the desired carbon content during atimeof one-half to one and one-half hours. A family of such curves can be,constructed from experimental data which willv enable the readydetermination of both time andtemperatures.

The effect ofmoisture during carburizing is most pronounced, althoughnotto quite the same extent as during sintering, satisfactory carburizingbeing difiicult to obtain with an atmosphere having. a dew point. above-.l5 F. Better a-nd'more consistent results are obtained byrnaintainingthedew point in the carburizing furnace at least between F. and F. oreven lower. Thislow dew point in the sinteringand carburizing steps is.easily obtained by passing commercial forms of pure hydrogen through-anoxidizing catalyst such as palladium to cause anyoxygen to react'withthe hydrogen and then over activated alumina or otherchemical dryingagents to remove the moisture thus formed. .Whenheated to the highcarburizing temperatures employed, the dew point will then be very low.If the carburizing furnace is constructed with an impervious iron orother metal muffle, the mufile can be put under a slight pressure withhydrogen and the exit gases ignited and burned oif at the furnace exit.This will prevent backing up of moisture into the muflle.

The time required for carburizing will depend both on the amount ofcarbon desired to be introduced, on the temperature employed, and on theratio of surface area to weight. As shown in the drawings, withtemperatures between 2050 F. and 2250 F., carburizing for one-half toone and one-half hours will usually sufilce to introduce 0.01 to 0.2%carbon. For the usual application with this type of high chromium alloy,the latter amount of carbon will be sufiicient. As indicated in thedrawings, the carburization may be carried further to obtain any desiredcarbon content.

The most convenient method of carrying out carburization is to pack thealloy compact in alu-ndum or other inert refractory material and placethe whole'in a graphite or carbon boat or other vessel. The loadedvessel then is placed in the furnace and heated.

The homogenizing step is carried out in a molten bath such as any of themolten heat-treating salts, such as barium chloride, borax, and thelike, molten glass and others. The exclusion of all atmosphere duringthis step prevents both oxidation and reduction, ensures efiicient heattransfer, and eliminates the difficulties attendant on controlling theatmosphere. The use of homogenizing or soaking temperatures above 2100F. permits a sufficiently short cycle to be practical and preventsubstantial grain growth. At these temperatures, the carbon freelydiffuses through the compact. The time required for this step willdepend on the time and temperature and also on the size and shape of thecompact and on the carbon content introduced during carburizing.However, practical commercial operations at 2100-2300" F. can beaccomplished in as little as one hour to as much as eight or twenty-fourhours. The use of homogenizing periods of two to ten hours usually issuflicient.

The powdered metal mixture utilized in this invention should contain atleast 11% and not more than chromium and may contain up to 2.5% nickelto obtain the desired heat-treatable, fatigue-resistant stainless steelcomposition. Small amounts of silicon (1% max.), sulfur, phosphorous,manganese, molybdenum, tungsten, titanium, columbium, zirconium, cobalt,and other alloying ingredients may be present, if desired, up to a totalof not more than 5% by weight. The chromium may be added in the purestate as ferrochrome or chromiumnickel alloy powder. The use offerrochrome and other chromium alloys is much preferred, not onlybecause of lower cost, but also because the intensely reactive chromiumis diluted and less apt to form carbides, oxides etc. during the initialsintering step. Ferrochrome containing from 67 to 72% chromium, 1%silicon (1.5% max), 1% carbon (max), sulfur up to 0.3%, phosphorous upup to 0.04% (max), and the balance iron is a commercial material foundentirely satisfactory. Similarly, the nickel may be added as a puremetal or as a high nickel alloy of iron or chromium. The use of ironalloys of these more reactive constituents, and especially of chromium,reduces the formation of a surface oxide coating.

The iron powder employed generally should contain less than 0.02% carbonand be very low in occluded, absorbed or oxide-form of oxygen, i. e.below 0.1%. In any case, the iron powder must be soft and have a highcompressibility. A suitable material of this type is electrolytic ironand any of the very pure commercial forms of iron powder. Any of theseforms of iron may readilybe deoxidized by heating in hydrogen for onehour at'about 1900 F., and if caking occurs, reground to a suitable meshsize. Regrinding and classification does v 6 not reintroduce muchoxygen. The iron preferably is finely ground and composed of uniformparticles, any powder having a sieve analysis between to 325 mesh orsmaller being satisfactory. Generally, the finer iron powders seem tofacilitate difiusion of alloying ingredients. The size of the-otheringredient powders should be comparable.

The metal powders may be mixed in any convenient manner to obtain athorough blending of the ingredients. The powdery mixture is then readyfor pressing.

The invention will now be described with reference to certain specificexamples which are intended to be illustrative only.

EXAMPLE 1 An iron powder known as Plast-Iron powder, a trade name, andhaving the analysis given below was mixed with 325 mesh ferrochromepowder (analysis below) in proportions to yield 12.5% chromium on thetotal mix.

Iron powder Analysis: Percent Iron 99.17 0 (hydrogen loss) 0.750 max.Carbon 0.020 Manganese 0.003 Phosphorous 0.005 Silicon 0.007 Sulfur0.005 All others 0.037

Sieve analysis Mesh: Percent Meta1 Powder Association standard sieveanalysis.

Ferrochrome Chromium 67-72% Silicon 1.50 max.

Carbon 0.75-1.00 Phosphorous 0.04 max. Sulphur 0.03 max.

Balance iron.

Before mixing with the ferrochrome the iron was heated in dry hydrogenat 1900 F. for one hour and reground in a Mikro-Pulverizer. Mixing ofthe two powders was continued for one hour. The resulting mixutre wasplaced cold in metal tensile specimen molds and compacted at- 70 tonsper square inch. Sintering of the tensile specimens was carried out in aGeneral Electric box-type high temperature electric furnace. Alow-carbon steel muffle extended through the furnace to project oneither side.

The ends of the muflle were closed in and provided with smallburner-like jet openings. A current of commercially-pure hydrogen,treated as described, and having a dew point below 25 F., was introducedunder a slight pressure and the jets on both ends of the mufiie ignited,and the muffle brought up to temperature. The compacts were then placedinside the muffie and heated for two hours at 2350 F., cooled and coinedat 70 tons per square inch.

At this point, the compacts were found to contain only 0.03% carbon asagainst a minimum of about 0.08% required for this 12.5% chromium ironalloy. The density of the compacts were found to be about 7.50 or 97% oftheoretical (7.75). The tensile bars were found to have a tensilestrength of 47,500 lbs./ square inch as compared to a literature valueof 50,000 lbs/sq. in. for a similar very low carbon wrought alloy.

When compacts of the above general composition were sintered in hydrogenof a higher dew point and higher 1 oxygen content, the specimens showedthe presence of an, oxide coating on some of theferrochrome particles.These compacts, .afterc'ompacting', sintering, coining andresiri'tering, failed to reach 95% of theoretical density and showedsome evidence of inadequate chromium difr fusion. This same effect isnot observed with soft iron nor to the same degree with lowerchromiumalloys containing less than 11% chromium. It appears, therefore,

that the furnace atmosphere is highly critical during the sintering ofhigh chromium alloys.

'Several of the compacts prepared above were carburized to increasetheir carbon content to the range of 0.08 to 0.15%. The compacts werepacked in alundum in closed graphite boats so that no metal cameindirect contact with the boat. The boats were heated for thirty minutesat 2150 F. in the mufile described above, with the hydrogen atmospherehaving a dew point below F;, then austenized for fifteenminutes at 1800F. and quenched in oil. The resulting heat-treated specimens developedthe following properties when'drawn at vari- It is readily apparent thatthe specimens prepared as above, by compacting, sintering, coining andcarburizing produce a heat-treatable stainless steel article having ahigh density. Fatigue-resistance of these specimens was very good. Asshown above, the physical properties closely approach those of a wroughthigh-chrome steel.

EXAMPLE 2 The addition of small amounts of nickel has quite asurprisingly large effect on the tensile strength of a 12.5% chromiumalloy, when made by the powder molding method of this invention. A 12.5%chromium mixture such as was utilized in Example 1 was modified by theaddition to separate portions thereof of, respectively, 0.5%; 1%; 1.5%;2% and 2.5% nickel powder. The resulting mixtures were compacted at 70't. s. i., "sintered for two hours at 2350 F; in a hydrogen atmosphereat 25 to 45 F. dew point, coined at 70 t. s. i., and resintered for twohours at 2350 F. The tensile strength of the as-resintered specimencontaining 2% nickel was about 94,500 lbs./ sq. in., a value double thatof 4748,000

for; a specimen prepared in the same manner but with y no nickel- Thedensity of these compacts was about 97% of theoretical.

When the compacts containing 2% nickel are packed in alundum in agraphite boat and heated for to 60 minutes at 2350 F. in a low dew pointhydrogen atmosphere, a heat-treatable product is obtained. After properheat-treatment in a low dew point atmosphere, tensile strengths of up to160,000 lbs./ sq. in. and increased hardness are, obtained.Homogenization of similarspecimens, but having a thickness of A inch ormore is readily accomplished by heating for up to 8 or 10 hours attemperatures of 2250 to 2350 F. After the homogenized specimens areheat-treated, they have a uniformly finegrained structure.

EXAMPLE 3 A cylindrical specimen of a composition similar 'tothose' ofExample 1 (containing 12.5 chromium and no nickel) is made by compactinga mixture of soft iron and ferrochrome. The diameter of the specimen is1 inch and the thickness /s inch. The mixture is first compacted at 70t. s. i., sintered for three hours at 2350 F. in a hydrogen atmospherehaving a dew point between 'and Rand finally coinedat 70 t. s. i. Theresulting'co-mpact, in the as-sintered condition, has a-carbon contentof about point below -25 F. for the prescribed time.

0.04% and adensity of: about 7.50 or between-96 and 98% oftheoretical.The weight and surface area of the compact is then determined and theweight of carbon per unit areaneeded'to produce a final carbon contentof 0.15%. is calculated. The value thus determined isused to determinefrom Fig.1 the time of carburization required at 2250 F. According toFig. 1, only about 45 minutes is required for this purpose. The sampleis then packed in alundum in a graphite boat and placed in the furnace,described above, having a hydrogen atmosphere and a dew The resultingcarburized compact is then immersed in a molten bath of. barium chloridefor three hours at 2100 F. for homogenization. Thecarburized,homogenized cylinder is found to be heat-treatable to yield a uniformfinegruined structure.

The stainless steel powder molding method of this invention is highlyuseful and has a number of outstanding advantages. In the first place,the method is the first powder molding method to produce aheat-treatable stainless steel article having physical propertiesapproaching those of a forged articleof the same or similar composition.Secondly, there is no need to control the carbon content during thepreliminary compacting and sintering steps since the carbon isintroduced after the article is compacted, sintered and coined. Withhigh chromium steels, precise control of carbon content is a mostdifficult operation in the presence of a reducing atmosphere. Thirdly,the low carbon content produces a higher density product than previouslyknown alloy powder molding methods. The obtaining of such high densitiesis believed due, in part, to the use of a highly compressible ironpowder, to the lowcarbon content, and the use of a'low dew pointreducing atmosphere during sintering. Fourthly, the method of thisinvention is believed to be the first powder method to be successfullyapplied to high alloy steels and-especially to-high chrome, stainlessalloys. The production of articles ofcomplicated shapes by powdermolding leads to large economies by elimination of expensive forging,machining, finishing and plating steps. Fifthly, the meth od produces afatigue-resistant article. of such a material is not possible with knownpowder molding methods as applied to stainless or high alloy steelsbecause of excessive porosity in the compacts. Other advantages residein the method including the production of high strength, hardness, highfinish and others.

The: heat-treatable, fatigue-resistant products made by the method ofthis invention require little further treatment for use in difiicultapplications. Usually, further machining, finishing, polishing orplating is not required. If an exceptionally high finish is required, asimple electropolishing treatment usually is sufiicient without platingor other treatments' The products can be heat-treated according to theusual techniques employed on the forged or wrought forms of the samealloy. Compressor blades for gas turbines, steam turbines, jet enginesand the like, when made by this method of this invention, approach theproperties of the best wrought and precision ground blades now in use.-

What is claimed is:

1. A method of;making ,a heat-treatable article of stainess steel whichcomprises compacting a mixture of between 11 and 25% of a powdered formof chromium and up to 2.5 of apowdered form of nickel, and balance lowcarbon, deoxidized; highly-compressible iron powder at a pressureabove50 tons per, square inch, sintering' the resulting compact at atemperature-above 2000 F. in a hydrogen atmosphere; having a dew point,below 25 F. to-decrease thecarbon content of the compact to'below 0.1%,coining, the resulting;sinteredcompact at a pres.-

sure above 60 tons per square inch, and carburizing saidv sintered andcoined compact at a temperatureabove2000. Frin ahydrogen atmospherehaving a dewpoint b'elow:

IS* F; and in the presence of, but out of contact with;

The production 2300 and 2450 F. in a hydrogen atmosphere having a 10 dewpoint below 25 F, to decrease the carbon content of the compact to below0.1%, coining the resulting sin-' tered compact at a pressure above 60tons per square inch, carburizing said siutered and coined compact at atemperature between 2100 and 2300" F. in a hydrogen atmosphere having adew point below -15 F. and in the presence of solid carbon until thecarbon content is in the range of 0.08 to 1.0%, and homogenizing saidcarbu- 10 rized compact in a molten atmosphere-free bath at atemperature between 2100 and 2300 F.

3. As a new article of manufacture, a product which is a heat-treatablepowder molded stainless steel article containing between 11% and 25chromium, up to 2.5 nickel, carbon in the range of 0.08% to 1% andbalance iron, which has a density of at least 96% of theoretical andwhich is prepared in accordance with the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS2,228,600 Hardy Jan. 14, 1941 2,397,533 Chevigny Apr. 2, 1946 2,489,838Webb Nov. 29, 1949 2,489,839 Witney Nov. 29, 1949 2,495,823 Rice Jan.31, 1950 2,527,521 Bloom Oct. 31, 1950

1. A METHOD OF MAKING A HEAT-TREATABLE ARTICLE OF STAINLESS STEEL WHICHCOMPRISES COMPACTING A MIXTURE OF BETWEEN 11 AND 25% OF A POWDERED FORMOF CHROMIUM AND UP TO 2.5% OF A POWDERED FORM OF NICKEL, AND BALANCE LOWCARBON, DEOXIDIZED, HIGHTLY-COMPRESSIBLE IRON POWDER AT A PRESSURE ABOVE50 TONS PER SQUARE INCH, SINTERING THE RESULTING COMPACT AT ATEMPERATURE ABOVE 2000*F. IN A HYDROGEN ATMOSPHERE HAVING A DEW POINTBELOW -25* F. TO DECREASE THE CARBON CONTENT OF THE COMPAFT TO BELOW0.1%, COINING THE RESULTING SINTERED COMPACT AT A PRESSURE ABOVE 60 TONSPER SQUARE INCH, AND CARBURIZING SAID SINTERED AND CINED COMPACT AT ATEMPERATURE ABOVE 2000* F. IN A HYDROGEN ATMOSPHERE HAVING A DEW POINTBELOW -15*F. AND IN THE PRESENCE OF, BUT OUT OF CONTACT WITH, SOLIDCARBON UNTIL SAID COMPACT CONTAINS CARBON IN THE RANGE OF 0.08 TO 1.0%.